1. Field of Application
The following description relates generally to telecommunications systems and wireless communications systems.
2. Prior Art
The capacity of a wireless cellular network has been limited by interferences. For example, a mobile user may experience excellent signal quality when it is very close to a base station. The signal quality quickly deteriorates, however, if the mobile user moves away from its serving base station and to the cell edge, where the signal from the serving base station is corrupted by the signals from neighboring base stations. Since within a cell there is more area to the cell edge than to the cell center, a mobile user is more likely to experience bad channels than good channels. Accordingly the average cell capacity can be a small fraction of the peak capacity achievable only at the cell center.
A set of neighboring cells in a cellular network can be considered as a multipoint broadcast system. The theory of Gaussian broadcast channels has established that by coordinating the transmissions of broadcasters, the capacity of the multipoint broadcast system approaches that of an MIMO (multiple input-antenna/multiple output-antenna) system, thus multipoint broadcasting in a wireless cellular network has the potential to increase the network capacity by many folds. Multipoint broadcasting has been adopted in LTE-A (Long-Term Evolution of 3rd generation of wireless cellular network, Advanced) and is referred to as CoMP (Coordinated Multi-Point transmission).
FIG. 1 illustrates an example multipoint broadcast system. Elements 102, 104, and 106 denote base stations 1, 2, and 3, respectively, and elements 152, 154, and 156 denote mobile stations 1, 2, and 3, respectively. Backbone 100 connects base stations 1, 2, and 3 to allow highspeed information exchange among the base stations. In FIG. 1, base stations 1, 2, and 3 form a set of collaborating multipoint broadcasters. Mobile users 1, 2, and 3 form a set of recipients. Base stations 1, 2, and 3, and mobile users 1, 2, and 3 together form a multipoint-broadcast set. Base stations 1, 2, and 3 transmit the combinations of the signals intended for mobile users 1, 2, and 3. At each base station, the combination “weight” for each mobile user signal can be different. Through elaborate algorithms, at each mobile user the desired signal in the received signal is enhanced whereas the undesired signal or the interference is cancelled or minimized. For example, when the transmitted signals from base stations 1, 2, and 3 arrive at mobile user 1, the signals for mobile users 2 and 3 are cancelled out or minimized, while the signal for mobile user 1 is maximized or enhanced, thus the signal quality of mobile user 1 improves significantly. Similarly, mobile users 2 and 3 will also see significant improvement in the quality of their respective signals. The combining of the different mobile-user signals at each base station is commonly referred to as “pre-coding”. The combining weight for each mobile-user signal at for each base station is an element in a so called “pre-coding matrix”.
The main difficulty in multipoint broadcasting is that it requires the full knowledge of the downlink channel (from base stations to mobile users) to be available at collaborating base stations for pre-coding. In an FDD (frequency-division duplex) network, the downlink channel information has to be fed back to the base stations by the mobile users via uplink channels. Typically, full channel feedback requires large amount of data, and the rapid channel changes often taking place in wireless channels require high feedback rate. These requirements lead to a feedback overhead that is often so large that little is left in uplink channel capacity for carrying useful data. In a TDD (time-division duplex) network, base stations can obtain the downlink-channel information from the uplink-channel information by channel reciprocity. Ideal channel reciprocity, however, exists only between the antennas of base stations and the mobile users. The transmitter (TX) and the receiver (RX) of a base station are generally different from those of a mobile user or other base stations. This introduces the differences in gains and phases between downlink and uplink channels, and destroys the channel reciprocity. Substantial efforts are needed to overcome the difference in the TX/RX chains to restore the channel reciprocity.
Without an effective approach to providing base stations the downlink channel information, the full potential of multipoint broadcasting will not be utilized in wireless cellular networks. Thus there is a strong need for a method, system, and apparatus that provide efficient downlink channel information feedback without diminishing the uplink channel capacity in FDD and without additional burdens to restore channel reciprocity in TDD.